This invention concerns piezoelectric materials and, more specifically, an improved piezoelectric composite material that comprises a piezoelectric ceramic powder dispersed in a polymer matrix.
A variety of electro-mechanical transducers such as hydrophones, air sensors, vibration sensors, pressure and stress sensors depend on the piezoelectric phenomenon exhibited by certain piezoelectric crystals, polarized ceramics, and polarized polymers and composites. An important class of prior art sensors have as their active sensing element solid shapes of piezoelectric ceramic materials. In the particularly important hydrophone application area, the piezoelectrically active ceramic component converts underwater sound pressure waves to electrical signals which are then amplified and displayed. In recent years large numbers of sensors have come to be used in wide aperture arrays of individual sensors.
The solid shapes of the active sensing elements are fabricated from piezoelectric ceramic powders by a variety of standard consolidation techniques. Electrodes are bonded to two opposing faces in order to polarize the piezoelectric material during fabrication and to sense the electrical signals that develop in the material during use. Lead zirconate titanate compositions which are collectively referred to as PZT materials, are widely used because transducers based on these materials exhibit moderate sensitivity and durability. Solid shapes in the form of cubes, plates, tubes, and arrays thereof are commonly employed.
The detecting sensitivity of such prior art dense solid piezoelectric ceramics (piezo-ceramic) sensors, however is modest. The sensitivity of a sound receiver material is characterized by a hydrophone Figure-Of-Merit (FOM) which is commonly defined as the product of two hydrostatic piezoelectric coefficients, i.e. a charge coefficient, d.sub.h, and a voltage coefficient, g.sub.h. For conventional PZT-based piezo-ceramics, typical values for d.sub.h are in the range of about 50-60.times.10.sup.-12 Coulombs/Newton (C/N) and values for g.sub.h are in the range of about 2-3.times.10.sup.-3 volt.times.meter/Newton (Vm/N). The product of these is an FOM with values in the range of 100-200.times.10.sup.-15 m.sup.2 /Newton (m.sup.2 /N). More sensitive hydrophone devices would result if piezoelectric materials with higher values of the d.sub.h and/or the g.sub.h coefficient were provided.
The hydrostatic charge coefficient d.sub.h is defined as the sum of the longitudinal charge coefficient d.sub.33 and two transverse charge coefficients d.sub.31 and d.sub.32. In the lead titanate (PT) and PZT families of piezoelectric materials the sum of the transverse coefficients (d.sub.31 +d.sub.32) is typically as large as the longitudinal coefficient and opposite in sign. Therefore d.sub.h in these materials is much smaller than d.sub.33. The present invention concerns piezoelectric composite materials that exhibit d.sub.h coefficients that more closely approach the d.sub.33 coefficient of the piezoelectric phase since the contributions of d.sub.31 and d.sub.32 are minimized.
Besides the use of solid shapes of piezoelectric ceramics, several types of piezoelectric composite materials have been investigated in which piezo-ceramics are combined with a polymer phase. The composites differ in the `connectivity` of the piezoelectric and polymer phases. It has been found that the piezoelectric properties of the composites depend strongly on the manner in which the individual piezoelectric and polymer phases of diphasic composites are interconnected. Each phase in a composite can be self-connected in zero, one, two, or three dimensions between parallel exterior surfaces. According to convention the `connectivity` of a two phase composite is given by two digits which refer, respectively, to the self-connectedness of the piezoelectric and polymer phases contained. For example, a composite comprised of piezoelectric powder dispersed in polymer is designated as having 0-3 connectivity. Diphasic composites can be fabricated in ten different connectivities. The 0-3 connectivity pattern is especially useful because it is a simple, effective material that can be fabricated in a variety of shapes including large area sheet and long cable forms.
The conventional piezoelectric materials of prior art dense ceramic sensors are compositions that have been optimized to have not only piezoelectric sensitivity but also strength and shock resistance in bulk forms. Lead zirconate titanate (PZT) compositions are most widely used. In order to provide piezoelectric ceramics that would have strength and shock resistance some compromises were made in piezoelectric sensitivity; that is, materials with adequate but modest piezoelectric coefficients came to be used.
Since 0-3 composites employ piezoelectric materials in powder form the requirement for strength and shock resistance in bulk ceramic forms no longer applies and new piezoelectric compositions with maximum piezoelectric sensitivities in composites are coming into use. The best of these new materials that has been previously described is the family of solid solutions of lead titanate (PT) and bismuth ferrite (BF) (referred to below as `undoped` PT/BF) that is described in copending U.S. patent application Ser. No. 742,334, filed June 7, 1985, entitled "Piezoelectric-Polymer Composites with 0-3 Connectivity for Transducer Applications". According to that disclosure, undoped PT/BF solid solution powders of any combination of the end members (lead titanate and bismuth ferrite) can be fabricated. The preferred compositions are those with 50-80% by weight of BiFeO.sub.3, which compositions are in the vicinity of the tetragonal/rhombohedral phase boundary of the composition.
The desired PT/BF composition is formed from the binary oxides: PbO, TiO.sub.2, Bi.sub.2 O.sub.3, and Fe.sub.2 O.sub.3. Appropriate weights of each oxide are mixed and ball milled for 12 hours and dried 8-12 hours in a low temperature furnace. This green mixture is milled in a spex for 10 minutes to break up agglomerations and further homogenize the batch and subjected to a low temperature (700.degree.-800.degree. C.) primary calcination for 1.5 hours, cooled, ground, sieved, and loosely compacted into pellets. The pellets are then subjected to a second high temperature firing (950.degree.-1050.degree. C.), from which they are quenched to room temperature either in air or by plunging into water. Quenching produces powders with average particle sizes of 5-10 micrometers depending on the specific composition.
PT/BF compositions exhibit a higher spontaneous strain on cooling below their Curie point (Tc). At temperatures above Tc PT/BF compositions exhibit cubic symmetry hence the `a` and `c` crystallographic are equal and spontaneous strain is minimal. On cooling below Tc PT/BF compositions adopt a tetragonal symmetry in which the `a` and `c` crystallograhic axes have sharply different values. Furthermore the c/a ratio is temperature dependant. The spontaneous strain in PT/BF compositions is among the highest for any piezoelectric material. This anisotropy in the crystallographic axes is the source of the high piezoelectric coefficients exhibited by these compositions.
The anisotropy and its temperature dependance are also the sources of strain within PT/BF crystallites. This limits the utility of these compositions in forming dense ceramic shapes but provides an excellent mechanism (i.e., thermal quenching) for forming fine powders of piezoelectric crystallites which can be dispersed and heavily loaded in polymers to form the composites of this invention.
Thermal quenching can be controlled to produce powders comprised of small diameter (1-10 m), equi-axed crystallites with narrow particle size distributions. The small polycrystalline aggregates that may form to some extent are readily dispersed as single crystallites during composite fabrication and poling.
Thermal quenching is preferred over conventional powder forming methods, as grinding, ball-milling, vibratory milling, etc., since composites made from powders prepared by conventional methods exhibit lower sensitivities to hydrostatic pressure. Extensive research directed at determining the origin of the lower sensitivity of conventionally prepared powders has led to the conclusion that structural defects are introduced in the microcrystals during the pulverizing process which leads to the formation of multidomained crystallites. It is a well known fact that it is more difficult to pole multidomained crystallites than single domained crystallites. That is, it is very difficult and, in some cases impossible, to completely develop a single polar axis oriented with an applied poling electric field in multidomained crystallites while this can be done in single domained crystallites. It is desired that the piezoelectric phase be an easily polable material since it will be used in polymer composites which are more difficult to pole than conventional dense piezoelectric ceramics. It is necessary that the piezoelectric phase be polable by electric field strengths that do not exceed the dielectric strength of the powder/polymer composite.
It is believed that thermal quenching produces a powder having crystallites with minimal defects. In aforementioned copending patent application Ser. No. 742,334, entitled "Piezoelectric-Polymer Composites with 0-3 Connectivity for Transducer Applications" a piezoelectric filler/polymer composite of 0-3 connectivity is described that employs a lead titanate-bismuth ferrite powder that is prepared by the thermal quenching technique to achieve higher hydrophone FOM's than achievable with prior art 0-3 composites.